WASHINGTON (CNN) -- Sen. Trent Lott, the No. 2 Republican in the Senate, intends to resign by the end of the year, the Mississippi Republican announced Monday.

Trent Lott, R-Mississippi, is serving his fourth term in the U.S. Senate.

"Trish and I have decided that it's time to do something else," Lott said, referring to his wife.

"Let me be clear: There are no problems," Lott said.

Lott, a four-term senator, said he lamented the GOP losing control of the House and the Senate after the 2006 elections and "some of the negativism that we're dealing with now" but that was not why he was leaving the Senate.Watch Lott explain why he is stepping down »

"I get up every day believing that I can maybe have a positive effect on what we do in the Senate," the Republican whip said. "I like being a happy warrior."

While announcing his decision to leave the Senate in Pascagoula, Mississippi, Lott said he made the choice to pursue a new course in life after listening to a church sermon on Ecclesiastes that says there is a time for everything.

"I don't know the future holds for us," said Lott, whose term would have expired in 2012.

Lott said he would continue to work on the issued he cared about until "the last day I will be alive."

"I still feel strongly about national defense. I still feel strongly about fiscal responsibility," Lott said.

Lott said he did not have a position lined up for him after he leaves the Senate, but sources tell CNN Lott plans to join the private sector. Lott also said he was considering teaching.

Two GOP sources close to Lott said they were not surprised by Lott's decision to retire.

"He's wanted to go into the private sector for a long time," one source said. "He stayed because of Katrina. Mississippi is doing very well, and he's delivered what he needed to deliver."

"He struggled hard with the decision to run for re-election. He decided he needed to work on more Katrina aid, but he's not fully enthused about another 6 years."

Republican sources close to Lott said one reason for Lott's decision to resign is the new lobbying restrictions on former lawmakers.

A law kicks in on January 1 that forbids lawmakers from lobbying for two years after leaving office. Those who leave by the end of 2007 are covered by the previous law, which demands a wait of only one year.

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"He wants to go make a living; he has no money," a GOP source said. "He'll be acting as a lobbyist and political adviser."

The source also said Lott wanted to leave now so that the person appointed to replace him would have almost a year in the Senate before facing the voters during the elections in November 2008.

Mississippi Gov. Haley Barbour is responsible for appointing someone to hold Lott's seat until a special election next year.

Rep. Chip Pickering, who has previously announced his resignation from the House, and Rep. Roger Wicker are both considered possible contenders for the seat.

Five other GOP senators -- John Warner of Virginia, Wayne Allard of Colorado, Pete Domenici of New Mexico, Chuck Hagel of Nebraska, and Larry Craig of Idaho have said they will not seek re-election next year.

Since Lott's Mississippi seat is considered safe for Republicans, GOP sources say Lott's resignations should not affect the Republicans' mood going into the 2008 elections.

"While the Republican mood is somewhat dark, this is not one of those examples of Republicans feeling they don't have good electoral prospects in '08," one source said. "Lott's decision is about Lott. This is just a changing of the guard and a safe seat, and Republicans have nothing to worry about."

Sen. Jon Kyl of Arizona, currently the No. 3 Republican in the Senate, will run for the whip position Lott is vacating, Ryan Patmintra, Kyl's press secretary, told CNN.

Senate Majority Leader Harry Reid, D-Nevada, praised the man who had been his counterpart when Lott served as the Republican leader.

"Senator Lott has been a true friend, consistently reaching across the aisle to serve the interests of the people of Mississippi and to help me serve the interests of the people of Nevada," Reid said in a statement. Senator Lott is one of the strongest defenders of the institution of the Senate and one of the most pleasant senators I have ever worked with."

The current Republican leader, Sen. Mitch McConnell of Kentucky, also praised his whip.

"Trent is the best whip I've ever worked with, and I'm pretty familiar with the requirements of the job," McConnell said in a statement. "I am deeply grateful for Trent's friendship; he is truly one of the great ones."

Lott's resignation will bring to an end his more than three decades in Congress. He won a House seat in 1972 and was first elected to the Senate in 1988.

He is the first person to serve as whip in both the House and Senate.

Lott, 66, served as Senate majority leader when Republicans controlled that body but was pushed out of the leadership post after he told a 2002 birthday gathering for former Sen. Strom Thurmond that the country would have avoided "all these problems" if Thurmond's 1948 segregationist presidential bid had succeeded.